78  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1905. 
IGNEOUS   ROCKS. 
Much  massive  granite  occurs  in  the  pre-Cambrian  areas  and  has  therefore  already  been 
mentioned.  Igneous  rocks  of  post-Paleozoic  age  are  also  abundant  and  in  many  cases  are 
important  because  of  their  intimate  connection  with  ore  deposits.  There  are  two  distinct 
groups  of  these  younger  igneous  rocks — the  intrusives  and  the  effusives — although  in  certain 
cases  intrusive  bodies  of  rocks  which  otherwise  occur  principally  as  flows  are  known. 
The  intrusives  comprise  granite  and  porphyries.  Granite  known  to  be  younger  than  Car- 
boniferous has  been  found  by  J .  B.  Richardson  in  the  Franklin  Mountains,  where  it  occurs 
along  faults.  In  the  Organ  Mountains  a  granite  cuts  Carboniferous  strata  and  at  its  periph- 
ery is  decidedly  porphyritic.  There  is  no  proof  that  these  granites  are  post-Cretaceous  and 
the  fact  that  pre-Cretaceous,  post-Carboniferous  granite  occurs  in  the  Bisbee  a  district  is  sig- 
nificant in  this  connection.  On  the  whole,  however,  it  seems  more  likely  that  the  Organ 
Mountains  granite  at  least  corresponds  in  age  with  the  porphyritic  intrusives  and  is  post- 
Cretaceous. 
The  porphyries  in  almost  all  localities  are  closely  related  in  appearance,  being  light-gray 
rocks  with  prominent  white  phenocrysts  of  feldspar.  They  range  in  composition  from 
granite-porphyry  through  syenite-porphyry,  quartz-monzonite-porphyry,  and  monzonite- 
porphyry  to  diorite-porphyry.  Intermediate  types  are  by  far  the  most  common.  What  may 
be  considered  i  he  represent  ativc  of  these  porphyries  is  practically  the  same  rock  that  occurs 
in  the  Clifton  copper  district.  These  rocks  are  of  post-Cretaceous  age.  Important  masses 
of  them  occur  near  Red  River;  at  Elizabethtown;  in  the  Cerillos,  Ortiz,  and  San  Pedro  moun- 
tains; at  Magdalena,  Santa  Rita,  Ilillsboro,  and  Hanover;  and  in  the  Burro  Mountains, 
the  Jarilla  Mountains,  and  the  Sierra  Blanca. 
The  flow  rocks,  which  are  the  youngest  igneous  rocks  of  the  region  and  cover  almost  all 
other  deposits,  comprise  rhyolites,  andesites,  dacites,  and  basalts.  The  rhyolites  and 
dacites  are  probably  most  abundant  and  in  places  attain  a  thickness  of  over  1,000  feet. 
These  rocks,  which  sometimes  contain  thin  interbedded  flows  of  basalt,  are  supposed  to  be  of 
middle  Tertiary  age.  The  latest  lavas  consist  of  sheets  of  basalt  and  are  widely  distributed 
throughout  the  territory.  Some  of  these  flows  are  so  recent  that  the  surface  is  practically 
unaffected  by  decomposition  or  erosion  and  presents  the  characteristic  broken  and  ropy 
appearance  of  very  young  lavas. 
The  older  flows  are  mainly  confined  to  a'broad  tongue  piercing  the  middle  of  the  Territory 
from  the  west  as  far  east  as  the  Rio  Grande,  although  rhyolite  is  also  known  in  the  vicinity 
of  Red  River.  The  younger  basalts  occur  in  many  places  over  the  northwestern  quarter  of 
New  Mexico,  in  various  localities  east  of  the  central  mountain  hell,  and  along  the  Mexican 
boundary  line  west  of  El  Paso. 
STRUCTURE. 
Of  the  four  factors  which  have  produced  the  present  topography  of  New  Mexico — effusion 
of  lavas,  sedimentation,  crustal  movements,  and  erosion — the  first  three  appear  at  first  sight 
to  have  been  by  far  the  most  important,  while  erosion  seems  to  have  simply  modified  the 
details  of  their  broad  effects.  But  more  careful  consideration  indicates  that  only  by  exten- 
sive erosion  have  the  effects  of  crustal  movements  been  exposed  and  that  erosion  has  pre- 
pared the  place  for  sedimentation  and  surface  flows.  Erosion  therefore  is  of  paramount 
importance. 
Deposition  of  sedimentary  rocks  is  responsible  for  the  broadest  physiographic  features  of 
the  plateau  province  and  the  great  plains  and  the  level  portions  of  the  desert-range  region. 
Lava  flows  have  in  most'eases  simply  veneered  over  the  surface  of  these  plains  of  sedimenta- 
t  on,  but  in  some  instances  have  mantled  and  leveled  surfaces  of  erosion. 
a  Prof.  Paper  U.  S.  Geol.  Survey  No.  21,  J904,  PI.  XII. 
